1. Field of the Invention
The present invention relates to a gas friction pump including stationary pump-active elements and rotatable pump-active elements cooperating with the stationary pump-active elements. The rotatable pump-active elements are secured on a rotatable rotor shaft. The rotor shaft and the rotatable pump-active elements form components of the rotor. The rotor shaft is supported by two, spaced from each other, radial bearings of which one is located closer to the center of gravity of the rotor, and by an axial bearing.
2. Description of the Prior Art
Gas friction pumps, in particular, turbomolecular pumps are successfully used in many applications where a low end vacuum needs to be reached.
The term turbomolecular pump follows from the similarity with turbines, i.e., with blade rings being alternative supported on rotor and stator. The pumping effect is produced by cooperation of the rotor and the stator and increases with an increase of the rotational speed of the rotor. There exist different concepts for forming the rotor and for supporting it. In one of the variants, the pump-active structure of the rotor is mounted on an end of a shaft. The structure can be formed, e.g., of discs into which the blades can be inserted and which then are shrinked on a shaft. Thereby, a disc package is provided on the shaft end. Beneath this package, the shaft is supported at two locations and is provided with a motor. This often is arranged between the two support locations. This type of support is called a floating support as the gravity center lies outside the support locations.
In another variant, the rotor is formed as one piece of a solid material with an inner hollow cavity. The “bell” is then reverse drawn with respect to the support and the motor, and is screwed with a located therein shaft. With this construction, the gravity center of the rotor lies in vicinity of or in the upper shaft bearing.
In both constructions, in particular with a floating support, the forces acting in the upper bearing are high. Therefore, the located there bearing should have a high stiffness.
The simplest variant of a support is a ball bearing. Because of large forces and rotational speeds in the upper bearing, oil lubrication and cooling are indispensable. Despite this, the service life of a ball bearing is limited, and a replacement should take place in regular intervals.
Because of the construction of the pump, a ball bearing, which forms the bearing located adjacent to the gravity center, lies very deep inside the pump. Therefore, the oil supply is difficult and the replacement expensive.
A further problem is created by hydrocarbon vapors which are released from the oil of the bearing. Those are pumped out during the operation of the pump, but during stoppage of the pump they can reach the pumped-out recipient. This is highly undesirable during the manufacturing of semiconductors.
In the state of the art, it is known to replace the ball bearings in the upper support with magnetic bearings. In order to achieve a desired stiffniess, as a rule, an active magnetic radial bearing needs to be used. The technical expenses associated with such a bearing are high: further, electric magnets are necessary which provide for a variable positioning (controlled by current). In addition, position sensors are needed which determine a position of a rotor and communicate in form of an input signal to control electronics. It controls current with which the electromagnets are drive. With an increased stiffness of the bearing, the current also increases. This generates additional heat that has to be removed. In comparison with ball bearings, noticeably higher technical expenses lead to high manufacturing costs, which are result of an increased demand for better maintenance and bearing characteristics.
In some application, e.g., in the semiconductor industry, magnetically supported pumps became acceptable. If one accepts, as conventional bearings, bearings a large number of which, when produced, is defective and which are available for many years on the market, magnetic bearings should be counted among them.
A different path was proposed already in 1973 in a laid-open German application DE-OS 22 55 618. This publication teaches use of gas bearings in turbomolecular pumps, wherein all degrees of freedom (except those of rotation) are supported by gas bearings. A high stiffness is achieved with gas bearings only when the clearance between the shaft and the stator of the bearing is very small. With this feature of reducing the clearance, one reaches the limits very quickly, in particular when the precision at mounting of stator and rotor parts is concerned. E.g., when two radial bearings, which are spaced from each other by a certain distance, are used, a smallest deviation from centering in one of the bearings leads to unpermissible change of the most narrow clearance in the respective other bearing, and it is very difficult if almost impossible to effect the necessary high-precision adjustments. For solution of this problem it was proposed, e.g., to appropriately suspend the bearing unit in the pump housing, as disclosed in DE-OS 199 15 983. The foregoing suggestion up to the present did not found commercial application in gas-supported turbomolecular pumps.
An object of the invention is to provide a floating or a bell-shaped rotor having a maintenance-friendly construction and a bearing of which located close to the gravity center of the rotor and has a high stiffness. The construction should solve the problem of a high-precision centering.